Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
Barium Chloride is represented as $\ce{BaCl2}$.
Since chlorine is a diatomic molecule, It should be denoted as $\ce{Cl2}$.
Formulating, we get
\begin{array}{|c:cc|}\hline \small \rm Element & \ce{Ba} & \ce{Cl2}\\ \small \rm Valency & 2 & 1 \\\hline \end{array}
Crisscrossing the valencies of Barium and Chlorine we get $\ce{Ba(Cl2)2}$ — as opposed to the accepted formula of $\ce{BaCl2}$. How is it so?
The compound barium chloride is not the same thing as barium and chlorine mixed together.
When they react, a barium atom will give up two electrons to form a action, and a chlorine molecule will pick up two electrons to form a pair of chloride ions: $$\ce{Ba -> Ba^2+ +2e^-}$$
$$\ce{Cl2 +2e^- -> 2Cl^-}$$ When you have both of those things at once, the electrons are "consumed" as fast as they are "produced", so they don't appear at all in the result: $$\ce{Ba +Cl2->Ba^2+ +2Cl^-}$$ which forms an ionic lattice when solid. Since this lattice has overall neutral charge, its ionic charges must balance with integer coefficients.
Funnily enough, these coefficients are $1$ and $2$ for $2+$ and $1-$ respectively, so these are applied to the ions which carry those charges. Thus: $$\ce{Ba1Cl2}$$ or more simply and directly: $$\ce{BaCl2}$$
When chlorine is in its free state it is diatomic. But when it reacts with barium it is not in the form of $\ce{Cl2}$. It will be in its ionic state which is $\ce{Cl-}$. The same goes for Barium. Barium is mono-atomic and its ionic state is $\ce{Ba^2+}$. Barium gives one electron to a chlorine atom and another electron to another chlorine atom, as valency of chlorine is 1, so it is $\ce{BaCl2}$.
